You might not think that a mother's breastmilk would vary much based on the sex of her offspring, but you would be wrong. It turns out a new study has found that when mothers in Kenya were poor, they produced fattier breast milk for their girls than they did for their boys. The roots of this research can be traced back to the seventies, when two evolutionary biologists, Robert Trivers and Dan Willard, introduced a hypothesis that said when men have multiple female partners, parents living in good conditions will invest more in boys, and when they live in poor conditions they'll invest more in girls. This has mostly been tested on a behavioral level, and this new study—led by Masako Fujista at Michigan State University—is one of the first to test the biological truth to this hypothesis.

The study was conducted using 83 moms in northern Kenya, where men are allowed to marry multiple women. They found that the poor women (defined by having less land and fewer livestock) had breast milk that contained more fat when they were nursing daughters than when they were nursing sons. Meanwhile, mothers who were in better financial circumstances produced fattier milk for their boys than they did their girls. This can likely be explained by the fact that when men have many wives, they need money to support that large family. A poor male is not going to be as appealing as a husband and is less likely to marry, and therefore he can't do anything to help his mother's status. A poor girl, however, is more likely to be profitable to her mother by marrying. So even when the children are babies, the poor mothers sense that they should invest more nutritional resources into girls than boys. The one piece of good news for the poor boys and rich girls in this scenario is that the study at least found that they didn't get fed any less frequently than the other babies did.